Polyolefins are widely used in demanding polymer applications wherein the polymers must meet high mechanical and/or electrical requirements. For instance in power cable applications, particularly in medium voltage (MV) and especially in high voltage (HV) and extra high voltage (EHV) cable applications the electrical properties of the polymer composition has a significant importance. Furthermore, the electrical properties of importance may differ in different cable applications, as is the case between alternating current (AC) and direct current (DC) cable applications.
Crosslinking of Cables
A typical power cable comprises a conductor surrounded, at least, by an inner semiconductive layer, an insulation layer and an outer semiconductive layer, in that order. The cables are commonly produced by extruding the layers on a conductor. The polymer material in one or more of said layers is then normally crosslinked to improve e.g. heat and deformation resistance, creep properties, mechanical strength, chemical resistance and abrasion resistance of the polymer in the layer(s) of the cable. In crosslinking reaction of a polymer interpolymer crosslinks (bridges) are primarily formed. Crosslinking can be effected using e.g. a free radical generating compound, such as a peroxide. Free radical generating agent is typically incorporated to the layer material prior to the extrusion of the layer(s) on a conductor. After formation of the layered cable, the cable is then subjected to a crosslinking step to initiate the radical formation and thereby crosslinking reaction. Peroxides are very common free radical generating compounds used i.a. in the polymer industry for said polymer modifications. The resulting decomposition products of peroxides may include volatile by-products which are undesired, since they can be hazardous and may have a negative influence on the electrical properties of the cable. Therefore the volatile decomposition products such as methane e.g. where dicumylperoxide is used, are conventionally reduced to a minimum or removed after crosslinking and cooling step. Such removal step is generally known as a degassing step. The degassing step is time and energy consuming and is thus a costly operation in a cable manufacturing process.
Also the used cable production line and desired production speed can bring limitations to the cable materials especially when producing power cables of a larger size. Moreover, i.a. the crosslinking rate and the crosslinking degree of the polymer in the cable layer should be sufficient in order to minimize or avoid any undesirable sagging problem occurring during the cable production, particularly when the cable is produced e.g. in a catenary continuous vulcanization (CCV) line (especially for thicker constructions), which is a well known vulcanisation line type in the field and described in the literature.
Electrical Conductivity
The DC electrical conductivity is an important material property e.g. for insulating materials for high voltage direct current (HV DC) cables. First of all, the strong temperature and electric field dependence of this property will influence the electric field. The second issue is the fact that heat will be generated inside the insulation by the electric leakage current flowing between the inner and outer semiconductive layers. This leakage current depends on the electric field and the electrical conductivity of the insulation. High conductivity of the insulating material can even lead to thermal runaway under high stress/high temperature conditions. The conductivity must therefore be sufficiently low to avoid thermal runaway.
Accordingly, in HV DC cables, the insulation is heated by the leakage current. For a specific cable design the heating is proportional to the insulation conductivity×(electrical field)2. Thus, if the voltage is increased, far more heat will be generated.
There are high demands to increase the voltage of a power cable, preferably of direct current (DC) power cable, and thus a continuous need to find alternative polymer compositions with reduced conductivity. Such polymer compositions should preferably also have good mechanical properties required for demanding power cable embodiments.